Extract nbnxm PairlistSets

Extract nbnxm PairlistSets

The class PairlistSets was part of nonbonded_verlet_t. This change
extracts it, moves it to pairlistsets.h and hides it from the rest
of the code. This requires a few more, trivial, methods in
nonbonded_verlet_t to operate on PairlistSets.
Also renamed the pairlist params struct to PairlistParams and
moved it to separate include and source files.

Change-Id: I8e728663e145ce1f8504ae7302d7208344cc7cf3